Laminated magnetic cores



Jan. 27, 1970 0.1-1. RYDER ET AL 3,492,617

' LAMINATED MAGNETIC CORES Filed Feb. 29, 1968 3 Sheets-Sheet 1 MAX 0 MAX F IG.3

FIG.2

Jan. 21, 197-0 D, H. RYDER ET AL 3,492,617

LAMINATED MAGNETIC CORES Filed Feb. 29, 1968 3 Sheets-Sheet 2 FIG.4

FIG. 5

Jam. 27, 1970 Hv R E ET AL 3,492,617

LAMINATED MAGNETIC CORES Filed Feb. 29, 1968 3 Sheds-Sheet 3 FIG. 6 54 FIG. 7

United States Patent 3,492,617 LAMINATED MAGNETIC CORES Donald Henry Ryder and John Thomas Wilkins, The English Electric Co., Ltd., Stafford, England Filed Feb. 29, 1968, Ser. No. 709,323 Claims priority, application Great Britain, Mar. 6, 1967, 10,369/ 67 Int. Cl. H01f 27/08, 27/24 U.S. 'Cl. 33660 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to laminated transformer cores.

Where stray flux from between the windings of a transformer enters the core generally at right angles to the plane of the core laminations large eddy currents can be generated so that at least the outer laminations of the core become overheated.

According to the invention there is provided a laminated transformer core having leg members interconnected by yoke members, at least two concentrically arranged cylindrical windings disposed about each leg member, each leg member comprising a plurality of laminations laid one upon the other and arranged in the order, a first packet of a plurality of narrow laminations, an intermediate packet of a plurality of wider laminations, at central packet of widest laminations, a further intermediate packet of a plurality of wider laminations and a last packet of a plurality of said narrow laminations, whereby each core leg is of symmetrically stepped cruciform cross section, the laminations of said first packet, said intermediate packet, said further intermediate packet and said last packet, being formed with a plurality of laterally spaced slots which extend longitudinally from the vicinity of the junction of a core leg with an adjacent yoke, the slots in said outer laminations being disposed relative to one another such that each part of a core leg formed by the slotted portions of the laminations is divided into a number of separate leg portions whereby to limit the paths of eddy currents which will flow in the planes of the laminations and which are generated by stray flux from between a pair of concentrically arranged windings and which enters the laminations normal or substantially normal to the planes thereof.

The slots may be graduated as regards their spacings, the slots being more closely spaced at the sides of the laminations.

A core leg may be built with similarly slotted laminations so that the slots in adacent laminations are in alignment with one another.

Alternatively a core leg may be built up with laminations having slots which are differently spaced so that the slots in adjacent laminations, or groups of adjacent laminations are staggered.

In order that the invention may be better understood slotted transformer core legs in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic plan view of the central leg of part of a transformer core with the top yoke removed to reveal one core leg in accordance with the invention,

FIG. 2 is a sectional side view of FIG. 1 on the line I-I,

FIG. 3 is a plot of the flux distribution along the length of the core leg shown in FIG. 2,

FIG. 4 shows slotted laminations which can be used to build a core leg in accordance with the invention,

FIG. 5 shows the lower part of a lamination similar to that of FIG. 4, but with slots having different lateral spacings which can also be used to build up a core.

FIG. 6 shows part of a core leg formed of slotted laminations; and

FIG. 7 shows part of a core leg, similar to that of FIG. 6, formed of slotted laminations but arranged to provide a differently slotted core.

FIG. 1 is a diagrammatic plan view of the central leg of part of a transformer core with the top yoke removed and shows the top of the central leg 10, the bottom yoke portions 11 and 12, a low voltage winding 13 disposed about the leg 10, insulation 14 disposed about the low voltage winding and a high voltage Winding 15 disposed about the insulation 14.

The leg 10 is clamped between a number of clamping plates 16 and a like number of oppositely disposed plates 17 which are of a non-magnetic, non-electrical conducting material. The actual clamping means is not important in explaining the present invention and is not therefore shown in detail. It should, however, be mentioned that the invention can be applied to core leg structures which are held together by other means such, for example, as by banding or bonding.

Leakage flux from between the two windings enters the core in the direction of the arrows A and B and flux in the direction of the arrows A causes the generation of large eddy currents with resultant excessive heating of the core. It will be noted that the stray flux A enters the core at, or generally at, right angles to the plane of the laminations.

The distribution of this leakage flux is shown in FIG. 3 and has peak values near the opposite ends of the windings, that is, near both ends of the leg member, and falls oil to a minimum at the extreme ends of the core and at the middle of the leg.

In order to minimize the eddy currents generated due to this leakage flux the leg is slotted as shown at 21 (FIG. 1) so as to divide up the eddy current paths at least at the points of maximum leakage flux. FIG. 1 shows a usual arrangement of packets of laminations forming a core leg of stepped cruciform cross-section in which the central packet of laminations is of maximum width and the core is stepped by building up on either side of the central packet, packets of laminations which are less wide so that the core is of cruciform cross-section, approaching circular cross section. FIGS. 1 and 2, which do not show the normal insulation barrier surrounding the core, or packing of the laminations used in the steps of the laminations, and in which latter FIGURE 2 no bottom yoke clamping or banding is shown, discloses a core in which the slots 21 extend into the leg for only the depth of the two outer packets of laminations. In building up a core in accordance with the present invention it is not necessary to provide slots through the Whole thickness of the core leg and the number of packets of the outer laminations which are slotted will vary from transformer to transformer and generally a core will be built up With rather more packets of laminations than shown in FIG. 1. As can be seen in FIG. 1, each of the laminations of the outer packets have five slots and the laminations of the wider next inner packets have seven slots. These two outer packets of laminations are slotted the whole length of the core between upper and lower yoke members. FIG. 2 shows the slots 21 extending to the bottom yoke member.

FIG. 4 shows the form of slotted lamination 20 which is used for the outer packets of laminations of the core leg. This lamination has five slots 21. In this case the slots extend from near the bottom end of the lamination to a point distant from the top end thereof since that portion of the lamination above the slots will be interleaved into the yoke of a transformer core. This form of lamination can thus be reversed so that with interleaved joints between the legs and yokes of the core the plain unslotted part of the lamination will be at the top and bottom part of the leg alternately. For the next wider packet of laminaions similar laminations with additional slots are provided.

It may not be necessary to form the core leg with all of the slots with the same spacing. Thus in order to avoid removing an unnecessary amount of the magnetic core material the core legs can be built up of packets of laminations which are slotted as shown in FIG. 5.

This FIG. 5 shows the lower half of a lamination 30 which is similar to, but wider than the lamination 20, and has seven slots 31 with the slots more closely spaced at the edges of the laminations than at the middle thereof.

Whilst the invention has been described in relation to FIG. 1 which shows all the slots 21 in alignment with one another the core leg can be built up of laminations having different forms of slotting so that the slots in adjacent laminations, or adjacent groups of laminations are staggered with respect to one another. Two such arrangements are shown in FIGS. 6 and 7.

FIG. 6 shows part of a core leg which is enclosed in an insulation barrier 53. This core leg includes laminations arranged in packets 50, 51, 52 in which the outer packet consists of four laminations each having eleven equally spaced slots. The next packet 51 consists of four laminations having seven unequally spaced slots as shown in FIG. 5, and the packet 52 consists of four laminations having five slots spaced as shown in FIG. 4. Adjacent the packet 52 are shown two further laminations comprising part of the next packet 54 of unslotted laminations.

The opposite side of the core leg (not shown) is formed of slotted laminations in a similar manner.

It will be seen that the arrangement of laminations of FIG. 6 provides a core leg which is divided into separate spaced portions so as to divide the leg in the plane of the laminations with the outer laminations of the leg divided by more slots than the inner laminations.

Thus the paths of eddy currents generated due to leakage fiux entering the laminations normal to the planes thereof are shorter in the outer laminations where the eddy currents would have the maximum heating effect and become progressively longer where the penetration of leakage flux becomes less.

FIG. 7 shows part of a core leg similar to FIG. 6 but with a different pattern of slotted laminations in which the outer packet 55 comprises four laminations with five slots spaced as shown in FIG. 4. In the next packet of laminations 56 the first two laminations are slotted with seven equally spaced slots. The next two laminations of packet 56 have unequally spaced slots as shown in FIG. 5 as have the first two laminations of the next packet 57. In the first two laminations of packet 57 the slots are more widely spaced since the laminations themselves are wider. The other two laminations of packet 57 have slots equally spaced as shown in FIG. 4.

In FIG. 7 the slots in the packets of laminations are shown as being filled with solid electrical insulating material as indicated at 58. However, the slots may be left unfilled as shown in the other embodiments of the invention, or only some of the slots filled with solid insulation. All unfilled slots may thus be used for the circulation of a cooling fluid. It will be seen that the slot pattern of FIG. 7 provides greater sub-division of the core leg at the edges of the lamination and also greater sub-division to a greater depth of the core than in the case of the FIG. 6 arrangement.

In other respects the modifications proposed for the FIG. 6 arrangement may also be employed in the FIG. 7 arrangement.

It will be obvious that using only the slotted laminations as particularly shown in the drawings, it is possible to choose or build up a slotting pattern of the laminations to divide up the core leg to achieve the best reduction of eddy current losses in relation to the intensity and distribution of the leakage flux around the core.

Core legs according to the invention may also be built with laminations having different numbers of slots.

If desired the slots formed in the core leg may be filled with solid non-magnetic electrically insulating material for strengthening the core leg and/ or for preventing movement of the slotted parts of the laminations. Filling of the slots may be particularly desirable where a core leg is banded since the banding with its inward radial pressure might cause the slotted laminations to distort such that the banding would be ineffective. However, as described in relation to FIG. 7, at least some of the slots may be left unfilled so that they may 'be used for the circulation of a cooling fluid.

It is usual to provide laminations with either an electrically insulating coating or a coating of material of relatively high electrical resistance so as to isolate the laminations from one another to reduce eddy currents. Laminations used in the present invention preferably have additional or more effective electrical insulation at least at the ends thereof so that the slots are not electrically bridged by adjacent laminations.

The use of additional insulation as described above is especially desirable where a transformer core has inter leaved joints for preventing bridging of the slotted leg laminations by adjacent yoke laminations.

Whilst the invention has been described in relation to a transformer core having laminations which are rectangular in plan, the invention can also be used in a core made of laminations having mitred ends.

We claim:

1. A laminated transformer core having leg members interconnected by top and bottom yoke members,

at least two concentrically arranged cylindrical windings disposed about each leg member,

each leg member comprising a plurality of laminations laid one upon the other and arranged in the order:

a first packet of a plurality of narrow laminations,

an intermediate packet of a plurality of wider laminations,

a central packet of widest laminations,

a further intermediate packet of a plurality of said wider laminations, and

a last packet of a plurality of said narrow laminations,

whereby each core leg is of symmetrically stepped cruci-form cross-section, the laminations of said first packet, said intermediate packet, said further intermediate packet and said last packet being formed with a plurality of laterally spaced slots which extend longitudinally from the junctions of the core leg with the top and bottom yoke members, the slots in said outer laminations being disposed relative to one another such that each part of a core leg formed by the slotted portions of the laminations is divided into a number of separate leg portions,

whereby to limit the paths of eddy currents which will How in the planes of the laminations and which are generated by stray flux from between a pair of concentrically arranged windings and which enters the laminations normal or substantially normal to the planes thereof.

2. A laminated transformer core according to claim 1, including at least one additional intermediate packet of a plurality of laminations disposed between the intermediate and center packets of laminations and at least one additional further intermediate packet of laminations disposed between the center and further intermediate packets of laminations, the widths of the additional packets of laminations being graduated whereby the crosssection of the core leg is substantially circular.

3. A laminated transformer core according to claim 2, in which at least some of the outermost, but not all, of the additional packets of laminations are formed with laterally spaced slots which extend longitudinally from the junctions of the core leg with the top and bottom yoke members.

4. A laminated transformer core according to claim 1, in which all the laminations of the first, intermediate, further intermediate and last packets are provided with similarly spaced slots whereby, in each divided portion of each core leg, the slots in one lamination are in alignment with the slots in the or each next adjacent lamination.

5. A laminated transformer core according to claim 1, in which the slots in the laminations of the first and intermediate packets, and in the further intermediate and last packets, are spaced differently from one another.

6. A laminated transformer core according to claim 1, including laminations in which the slots are more closely spaced near the outer sides of the laminations than near the longitudinal center line thereof.

7. A laminated transformer core according to claim 1, in which at least some of the slots between the separate leg portions are filled with a solid non-magnetic electrically insulating material for providing mechanical support to the separate core leg portions.

8. A laminated transformer core according to claim 1, in which at least the slotted parts of the laminations are electrically isolated from adjacent laminations.

References Cited UNITED STATES PATENTS 1,546,885 7/1925 Burnham 33660 2,382,172 8/1945 Putman et al. 336-2l9 XR 2,947,960 8/1960 Fredrickson 336-219 XR 2,962,679 11/ 1960 Stratton 336-234 XR 3,173,113 3/1965 Benke 336-234 XR THOMAS J. KOZMA, Primary Examiner US. Cl. X.R 3362l9, 234 

